It is widely used to detect the presence or absence of liquid in an internal space of a tank or a pipe during production of industrial products or general use of various devices. In the related art, a so-called transmissive liquid detection device is disclosed. Further, in the related art, a so-called capacitive liquid detection device is disclosed.
In the transmissive liquid detection device, however, it is required to form a part of the tank or pipe with a transparent material through which detection light can transmit. In addition, when detecting the presence or absence of a liquid at a plurality of positions, the capacitive liquid detection device is required to ensure a sufficient distance from an adjacent liquid detection device in order to avoid interference of the adjacent liquid detection device.